Frequently, fabrication processes require that a machining operation be performed at, or very near to, the center of one surface of a fragile or brittle workpiece. An exemplary machining operation is the cleaving of pieces of crystals of hard materials such as lithium fluoride, LIF, into flat blanks. Like diamond cutting, this operation as presently performed is time-consuming, requiring considerable skill and patience on the part of the technician. Cleavage of each hard crystal workpiece must occur along a plane at half the thickness of the crystal so that parting forces will be equally balanced against both sides of the crystal. To obtain one crystal blank of a desired thickness from a larger piece of crystal, the larger crystal piece must be subjected to multiple cleaving operations in which the crystal piece is bisected into successively thinner slabs. When the thickness of a LIF crystal workpiece, for example, is reduced to less than 2 millimeters, balance and control of the applied parting forces are critical to avoid crystal breakage. Present machining operations require that a fragile or brittle workpiece be carefully measured, clamped in a workpiece holder or jig, and the workpiece holder painstakingly centered relative to a tool such as a drill bit or a cleaving blade held by a machine.
Workpiece centering vises are available for general machine shop use for holding workpieces which are larger in size, of a more ductile composition and durable structure than brittle workpieces such as lithium fluoride crystals.
Typically, centering vises have two oppositely movable jaws for holding a workpiece in compression. The separation between the jaws is usually adjusted by rotation of a single central screw having right and left-hand threads at opposite ends. Workpiece centering vises are particularly useful in repetitious fabrication processes where the dimensions of similar workpieces tend to differ significantly because,if accuracy and precision are not important, a single, preliminary orientation of the vise relative to a machine tool,for example, eliminates the necessity of continuously re-orienting the vise to accommodate variations in size between subsequent workpieces.
One type of presently available centering vise has an open frame supporting a single screw with right and left hand threads at opposite ends. A pair of facing jaws with central threaded openings engaging oppositely threaded sections of the screw, rest on the sides of a frame. Separation between the jaws is controlled by turning the single screw. Another state of the art centering vise has the sides of both jaws supported by a lathe bed machined into the bed-frame of the vise. Although presently available workpiece centering vises provide some measure of saving of set-up time in repetitious fabrication processes, they are not entirely suitable for handling small workpieces of fragile or brittle materials like hard crystals, particularly during such delicate processes as crystal cleaving. Delicate workpieces such as those of brittle materials, have a tendency to shatter if subjected to more than very slight compression when held between the jaws of a vise. Vise jaws supported by linear surfaces such as lathe beds or the sides of a bed-frame drag against their supporting surfaces when adjusted, thereby creating frictional forces which impede adjustment of the jaws via the screw and delaying tactile perception of minute adjustments in the jaw separation. This prevents such vises from being repetively operated to a close degree of precision. Additionally, it is quite easy for a technician using presently available vises to unwittingly move the jaws too close together and crush or shatter a fragile or brittle workpiece.
Brittle workpieces such as lithium fluoride crystals must not only be precisely centered during a cleaving process, but one axis of the crystal must be oriented parallel with the blade of a cleaving machine. If the drag between both sides of the jaws and their supporting surfaces is not continuously equal, the jaws tend to wobble when adjusted. Consequently, opposite jaw faces may engage different ends of the workpiece and either hold it skewed relative to a machine tool or subject it to a destructive compressive couple. If the workpiece is a thin, brittle crystal this may either cause the workpiece to be shattered or to be cleaved at an angle to its plane of crystalization, thereby resulting in cleaved crystal blanks with irregular, stepped rather than smooth surfaces.
Furthermore, wear between the adjusting screw and the vise frame introduces axial end-play in the screw which reduces the ability of the vise to provide precise centering of successive workpieces. The only way provided by presently available workpiece centering vises to accommodate for such end-play is frequent reorientation of the vise relative to the machine tool. Reorientation, however, is time consuming and not completely satisfactory because it only partially compensates for end play but does not eliminate it. It is apparent, therefore, that the absence of precision and the delay in tactile perception render state of the art centering vises unsuitable for use with brittle workpieces of materials such as hard crystal, particularly where quick set-up and repetitive precision are desired.